1. Field of the Invention
The invention relates to a drive system for a printer, in particular for a matrix printer of a needle-type construction or, respectively of a hammer-type construction, where an electromagnetic coil is coordinated to each printing element.
2. Brief Description of the Background of the Invention Including Prior Art
The electromagnetic coil of a print needle (needle print head) or, respectively of printing hammers (hammer bank printer) is furnished with current during operation in a matrix printer in order to induce and launch the needles or, respectively, the printing hammers, to image characters on the recording carrier via individual dot prints.
The printing element can comprise, in each case, a spring loaded magnetizable armature disposed opposite to the electromagnet core, or, respectively for hinged clapper armature print heads with rigid armature, with, in each case, a print element attached to the armature, and with a permanent magnet. The permanent magnet is permeated in this sense, upon switched-on electromagnetic coil, by the magnetic field of the electromagnetic coil, or a mechanical spring, in the case of a hinged clapper armature print head, that the permanent magnet or, respectively, the mechanical spring, in case of a switched-off electromagnetic coil, retains the spring-loaded armature or, respectively, the rigid armature in a rearward launching position, and that, in case of an electromagnetic coil with a current flow, the magnetic field of the permanent magnet is balanced, or, respectively, the force of the mechanical spring is overcome, and that thereby the spring-loaded armature or, respectively, the rigid armature, is launched or fired with the print element in a forward disposed print position. A current-limiting circuit is employed, where a rapidly rising current is limited via a relatively high voltage, and with a current flank decaying after a first pulse time to zero, as well as with a capacitor storing electrical energies.
The frequency of the system, which includes an electromagnetic coil, a metallic magnet core, an armature spring, as well as a needle attached to the armature spring, in connection with the construction of a character matrix, is directly proportionate to the obtainable writing speed of the printer. The frequency of the system is determined by the time, which the needle requires to pass from the rest position into the working position and back again to the rest position in order to be ready for the next required dot print. This frequency of the magnet system in turn is again very dependent on the time, which is required in order to remove the voltage at the electromagnetic coil. The occurring difficulties during the driving of a magnet system comprise that the current, which passes through the electromagnetic coil during the actual driving time, does not break off immediately after the removal of the drive voltage. The inductivity of the electromagnetic coil tends to maintain the current in the existing direction. Because of this, the voltage rises at the switched-off connection of the electromagnetic coil, as noted in the German Patent DE-PS 31 39 502.
A current-limiting circuit within driver circuits for printers, together with a character-generator or pattern generator, creating print signals and start signals, is known from the German Patent Application Laid Open DE-OS 31 51 242. The print signal and the start signal are connected at the signal input via AND-gates. Furthermore, in each case, monostable trigger circuits are provided and connected to the AND-gates, which trigger circuits, in each case, comprise a time-delay member or a timing element. The outputs of the monostable trigger circuits are separately connected via selection drive gates with the base of the drive transistors. The emitters of the drive transistors are connected via base resistors, in each case, to the base of transistors. The collector of the transistors or, respectively, the emitters of the transistors can be switched at voltage stages. The driver magnet coil for the needles or, respectively, the hammers to be fired, is provided at one of the transistors. Such current-limiting circuits maintain the current at a predetermined level in the end region of a time period of increasing current and are switched off after a time of uniform constant current strength such that the current course at the driver magnet coil forms approximately a non-isosceles trapezoid.
The system, electromagnetic coil-permanent magnet, effects in principle a higher frequency at the matrix print head of a hinged clapper armature construction, such that a constant development of quicker printing print heads could be created. The frequency of such print heads runs about 2 kHz and frequencies of from 3,500 to 4,000 Hz are desired.